The present invention relates to a polyorganosiloxane composition. More particularly the present invention relates to room temperature curable polyorganosiloxane compositions that are rendered stable under moisture free hermetically sealed closed conditions and which cure upon contact with water present in ambient air at room temperature to form an elastomeric composition.
Among polyorganosiloxane compositions that cure at room temperature to form elastomers, one-package or one -part types that cure upon contact with water, however the water is delivered, have been widely used as elastic adhesives or coating agents in the electrical and electronic industries. Also such compositions have been used as sealants in the construction industry since they necessitate none of the troublesome steps involving the measuring and weighing of the quantities of silicones, cross-linking agents, catalysts and mixing and preparing the sealant. Because one-part compositions avoid these steps, the materials are convenient to use and reproducible in their properties.
These one-part compositions comprise a silanol terminated polydiorganosiloxane and a cross-linking agent having more than two hydrolyzable groups per molecule. Upon curing, they generally release acetic acid, a long chain carboxylic acid, an organic amine, an organic amide, an organic hydroxylamine, an organic oxime, an alcohol or acetone depending on the chemical structure of the cross-linking agent employed.
A composition that releases acetic acid, while it generally possesses excellent adhesion and cures quickly, has an offensive and irritating odor. Further, the acetic acid released is frequently corrodes metals when the sealant is used to create a seal between a metal and another substrate. While the odor problems are not as significant with long chain carboxylic acids, the corrosion problems still occur because of the carboxylic acid functionality that is present. Compositions that cure by means of releasing an amine present toxicity issues as well as causing odor. Compositions that cure through the release of hydroxylamine, oximes or amides still corrode metals because these types of compounds are known metal ligands and form complexes with susceptible transition metals such as copper, which is one mechanism of corrosion. Compositions that cure by releasing acetone suffer from yellowing and the cross-linking agents are not convenient to prepare.
Room temperature vulcanizable compositions that cure by releasing alcohols offer a certain convenience because alkoxysilanes are used as the cross-linking agents. The compounds released by curing are alcohols and these generally present no odor or toxicity problems. Alkoxy curing compositions (alcohol releasing RTV compositions) do suffer from a slow cure rate and are very easy to hydrolyze, that is very small amounts of water trigger the curing reaction. This leads to problems with storage of the compositions.
Non-corrosive, alkoxy functional, one-component RTV silicone sealants can be made from polyalkoxy terminated polydiorganosiloxane polymers, alkoxysilane crosslinking agents and Sn(IV) catalysts. As disclosed in U.S. Pat. No. 4,517,352 ('352) and U.S. Pat. No. 4,395,526 ('526), such compositions do not exhibit long term storage stability, i.e. they are not shelf stable.
Shelf-stability as applied to one-component alkoxy curing RTV compositions means a moisture curable mixture wherein the uncured and cured properties are substantially unchanged relative to a freshly prepared mixture after being stored in a moisture resistant and moisture free container after accelerated aging for 24 hours at 100.degree. C. Alternatively shelf stability means a moisture curable mixture wherein the uncured and cured properties are substantially unchanged relative to a freshly prepared mixture after being stored in a moisture resistant and moisture free container for an extended period of time at ambient conditions, e.g. one year or two years.
In one-component RTV mixtures, free alcohol, typically methanol (CH.sub.3 OH), degrades the polymer in the presence of Sn(IV) catalysts. Over time this degradation leads to a failure of the mixture to cure. The solution to this problem is presented in the '352 and '526 patents where it is taught that a hydroxy reactive silane is necessary to scavenge free alcohol preventing polymer reversion thereby increasing shelf life.
A variety of mono-, di-, and tri-functional amino silanes and silazanes as alcohol (methanol) scavengers are disclosed in U.S. Pat. No. 4,593,085 ('085); U.S. Pat. No. 4,417,042 ('042); U.S. Pat. Nos. 4,395,526; 4,720,531 ('531) for use with methyldimethoxy endcapped polydimethylsiloxane polymers to give stable, fast curing, room temperature vulcanizable compositions. The preferred amino functional silanes and silazanes and their derivatives disclosed in these patents, e.g. methylaminosilanes, ethylaminosilanes, propylaminosilanes, butylaminosilanes, hexylaminosilanes and hexamethyldisilazane, are corrosive to copper bearing alloys. The low molecular weight amino by-products of the alcohol scavenging reaction cause contact and vapor corrosion when copper (or another susceptible transition metal) is present in the substrate alloy. Other one-component formulations that cure by release of acetic acid also cause corrosion, primarily because of acid attack on the metal. While titanium catalyzed one component formulations curing by release of an alcohol, (alkoxy based formulations), such as disclosed in U.S. Pat. No. 3,294,739 ('739); U.S. Pat. No. 3,334,067 ('067); and U.S. Pat. No. 3,708,467 ('467), are non-corrosive they are slower to cure, have a shorter shelf life and in some instances require a primer. Titanium catalyzed compositions are non-corrosive because the titanium catalyst decomposes to form relatively inert titanium oxides (rutile and anatase).
U.S. Pat. No. 4,537,944 ('944) and U.S. Pat. No. 4,760,123 ('123) disclose the use of certain compounds as alcohol scavengers that mitigate some of the problems associated with alkoxy based formulations. These patents disclose alcohol scavenging compounds that contain a silicon-nitrogen bond and when reacted with alcohol form a non-volatile nitrogen containing compound. Thus, these types of compounds also tend to reduce the amount of corrosion observed when room temperature vulcanizable sealants are used to seal metals because vapor corrosion is correspondingly reduced. The silicon nitrogen compounds that have been found to be particularly useful are those containing a silicon nitrogen bond wherein the nitrogen is bonded indirectly to the same or another silicon by or through one or more carbon atoms. While the '234 and '492patents disclose and describe the use of non-corrosive alcohol scavengers derived from the reaction product of hexamethyldisilazane and aminoalkoxysilanes, the yield for this reaction is below 50 mole % and the major reaction product, 1,1-dimethoxy-2-trimethylsilyl-1-sila-azacyclopentane is highly toxic. These compounds are typically prepared by the following reaction scheme: ##STR1## which reacts further to generate the active oligomeric species: ##STR2## albeit in yields significantly below 50 mole % based on the starting aminoalkoxysilane. A significant problem associated with these compounds is the very toxic nature of the 1,1-dimethoxy-2-trimethylsilyl-1-sila-azacyclopentane: ##STR3##